Polyurethane foams formed by the reaction of a polyisocyanate with a polyhydroxyl-containing compound in the presence of a suitable catalyst are widely accepted as padding materials for cushions in furniture, automobiles and the like.
Polyurethane foam formulations typically require a blowing agent to generate the gas to fill and expand the polyurethane foam cells. The resultant density of a polyurethane foam is controlled by the quantity and efficiency of the blowing agents. While carbon dioxide may be generated as a blowing agent through the reaction of water with the polyisocyanate, the use of low-boiling inert liquids, in particular, chlorofluorocarbons (CFCs), to augment or replace the chemical blowing action, has led to certain property advantages in the final foams, such as improved softness and higher elasticity.
However, the CFCs used as blowing agents and for other purposes are now suspected to be linked to the depletion of ozone in the upper atmosphere where the generally inert CFCs are decomposed by ultraviolet light. To avoid this potential problem, polyurethane foams blown only with carbon dioxide have acquired renewed interest.
Flexible polyurethane foams have been manufactured for more than thirty years from organic polyisocyanates and polymeric polyol using water as the primary blowing agent. Until recently, the polyisocyanate most commonly used in the production of flexible foams has been tolylene diisocyanate (TDI), but recent years have seen an increasing use of diphenylmethane diisocyanates (MDI), especially 4,4'-MDI, 2,4'-MDI, 2,2' MDI and polymers thereof.
However, water blown flexible polyurethanes produced heretofore have problems associated therewith affecting the stability thereof. These problems need to be overcome in order to produce a useful product. For example, the use of carbon dioxide generated from the reaction of water with isocyanate as a sole blowing agent necessitates formulations containing relatively large amounts of water to obtain low density foam grades; this increase in water makes the foam unstable.
One method of making the foam more stable is to increase high functionality (f.gtoreq.3) polyisocyanate oligomers of the MDI series in the polyisocyanate composition during processing. These higher functionality oligmers ("polymeric MDI" oligomers) produce much crosslinking in the molecular structure of the foam polymer. This helps prevent the foam from collapsing or recessing during rise. The need for this stabilization increases as the foam density decreases. Unfortunately, the added crosslinking detracts from desired mechanical properties and processing characteristics of MDI flexible foam. It causes the foam system to build viscosity faster and gel earlier in the reaction profile (i.e., at lower % conversion of --NCO groups)--thereby hindering the ability of the reacting system to "flow". Poor flow causes difficulties in filling complex molds, especially at low foam densities. The added crosslinking also reduces ultimate elongation and tear strength of the final foams.
The foams can also be made stable by increasing the EO ("ethylene oxide") content of the polyols in the isocyanate reactive composition. However, this increases compression sets, produces tight foams and reduces flow. Yet another way to stabilize the polyurethane foam is via the use of highly potent surfactants. However, this would result in poor fatigue, low resilences and high compression sets.
It is therefore the object of the present invention to develop a low density flexible polyurethane foam which does not suffer from the problems discussed hereinabove.
It is additionally an object of the invention to achieve stability (in low density systems) with reduced crosslinking, in particular, by using lower levels of the higher functionality (f.gtoreq.3) "polymeric MDI" oligomers in the polyisocyanate component. This object is directed primarily to all-MDI based, all-water blown (CFC free) flexible foams of densities less than 3.0 PCF, but greater than 1.6 PCF.
Another object of the invention is to achieve improved mold flow (mold filling ability) without increasing mold residence time. This object must be achieved within the design limits of two-component foam processing machines now in wide use.
Yet another object is to improve physical properties such as elongation and tear strength of the foams, without compromising other important physical properties, such as compression-set resistance.